Various particular embodiments include an integrated circuit (IC) structure having: a stack region; and a silicon substrate underlying and contacting the stack region, the silicon substrate including: a silicon region including a doped subcollector region; a set of isolation regions overlying the silicon region; a base region between the set of isolation regions and below the stack region, the base region including an intrinsic base contacting the stack region, an extrinsic base contacting the intrinsic base and the stack region, and an amorphized extrinsic base contact region contacting the extrinsic base; a collector region between the set of isolation regions; an undercut collector-base region between the set of isolation regions and below the base region; and a collector contact region contacting the collector region under the intrinsic base and the collector-base region via the doped subcollector region.